Well, when you are standing, the distance the blood has to travel is all up (from feet back to heart). Your heart will then have to accomodate by pumping faster. When you are sitting, the vertical distance decreases and your heart can relax a bit. Also, when you are standing your heart is positioned higher up from your feet, compared to when you are sitting.

About the atheletes... they would have been doing much more exercise than the normal person, and the organs would therefore adapt to that life style.

Staff: Mentor

Digital Limit said:

Why must an athlete exercise harder and longer to achieve a maximum heart rate and blood pressure as compared to an average person?

That's kinda what it means to be in shape. It makes more sense to look at it from the other direction: a person who is out of shape gets tired faster and with less exertion than a person who is in shape.